The purpose of this project is to assess the relative contributions of genetic and environmental factors to alcohol drinking and response to alcohol challenge. The project is composed of two studies. In the first, 100 male monozygotic (MZ) and 100 male dizygotic (DZ) twin pairs will complete a questionnaire describing their alcohol and other drug use and a 28-day record of their alcohol intake. These data will permit estimation of the relative contributions of genetic and environmental factors to frequency and amount of alcohol consumption. In addition, self- reports of contact between cotwins will permit estimation of the importance of shared environment on cotwin similarities in alcohol drinking. Finally, comparison of the questionnaire estimates of alcohol consumption with those derived from the 28-day record will permit evaluation of the validity of the questionnaire method. In the second study, 15 male MZ and 15 male DZ twin pairs will receive, in separate testing sessions, a placebo and two doses of alcohol (0.40 and 0.80 g/kg of 95% ethyl alcohol). The protocol will consist of electrophysiological measures (e.g., brainstem auditory evoked responses, resting EEG, and visual and auditory event-related potentials), self-reports of affect, and a measure of standing stability. The use of placebo and multiple doses of alcohol will permit conclusions about the effects of alcohol on information processing, response production, mood, and motor activity. The twin design will provide information on the relative contributions of genetic and environmental factors to variability in these measures in the drug-free state and following response to alcohol challenge. Finally, conclusions regarding the stability of the measures across time will be based on comparisons of baseline measures across the three sessions.